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Venus: highly radioactive or just cooling down?

The surface temperature of Venus is about 480°C, higher than any other solar system planet. While Venus does trap solar radiation in its atmosphere greenhouse fashion, data from Pioneer Venus Orbiter show that the planet radiates 15% more energy than it receives from the sun. In other words, Venus's surface is hotter then it would be if only the greenhouse effect were operating. Where could this extra energy come from? If it arises from the decay of naturally occurring radioactivity, Venus would have to have 10,000 times as much radioactivity as the earth. If this is the case, Venus must have had an origin radically different from the earth's.